Method of forming a cathode ray tube screen

ABSTRACT

A METHOD OF PATTERN FORMING FOR COLOR TELEVISION PICTURE TUBES WHICH COMPRISES COATING THE FACE PANEL OF A TUBE WITH A SLURRY OF A PHOTOSENSITIZED MATERIAL AND PHOSPHOR BY UTILIZING THE FORCE OF GRAVITY AND THEN EXPOSING THE COATED PANEL TO A SUITABLE SOURCE OF RADIATION THROUGH A NEGATIVE AND DEVELOPING TO FORM A PATTERN.

Oct. 24, 1972 ETAL 3,700,444

METHOD OF FORMING A CATHODE RAY TUBE SCREEN Original Filed April 5, 19672 Sheets-Sheet 1- v INVENTORS JOHN J'. MIL 45/? BY 7mz pzus 1/. RYcmmsK/ Oct. 24, 1972 J LL R HAL METHOD OF FORMING A CATHODE RAY OriginalFiled April 5. 1967 TUBE SCREEN 2 Sheets-Sheet 2 v INIVENTORS Jam JM/LLER BY TWIDMz/s 1/. FYCWMWSK/ ATTOR/Vf V United States Patent O3,700,444 METHOD OF FORMING A CATHODE RAY TUBE SCREEN John J. Miller andThaddeus V. Rychlewski, Seneca Falls, N.Y., assignors to SylvaniaElectric Products Inc. Continuation of application Ser. No. 627,745,Apr. 3, 1967. This application Feb. 10, 1970, Ser. No. 9,119 Int. Cl.H013 9/20, 31/20 US. Cl. 96-361 1 Claim ABSTRACT OF THE DISCLOSURE Amethod of pattern forming for color television picture tubes whichcomprises coating the face panel of a tube with a slurry of aphotosensitized material and phosphor by utilizing the force of gravityand then exposng the coated panel to a suitable source of radiationthrough a negative and developing to form a pattern.

This application is a continuation of application Ser. No. 627,745 filedApr. 3, 1967 and now abandoned.

BACKGROUND .OF THE INVENTION This invention relates to methods offorming cathode ray tube screens and more particularly to methods offorming mosaic patterns on screens suitable for use in color televisionpicture tubes. It is partcularly useful when used in conjunction withthe so-called direct photographic method of screen deposition.

In forming patterned screens in this manner", a prior method utilizes anaqueous medium contaning, in suspension, phosphor particles, an organicpolymerizable binder such as polyvinyl alcohol, and a photosensitizersuch as ammonium dichromate. A quantity of this mixture, or slurry, isdeposited near the center of a cathode ray tube face panel, which has agenerally concavo-convex configuration and is positioned with theconcave side uppermost, and is then spread thereover, by the action of aconstantly varying force generated by tilting and spinning the panelaccording to a selected and rather complicated schedule. After theslurry has been spread over the panel to provide a layer ofsubstantially uniform area density, it is dried. The phosphor coating onthe panel is then exposed through a suitable negative providing adesired pattern, to a source of radiaton which is capable ofpolymerizing the slurry. Usually this radiation is actinic. The exposedlayer is then developed by washing it to remove the unexposed slurry,thus leaving the desired pattern of elemental phosphor areas (dots orstripes) adhered to the panel. The process is repeated for each of aplurality of desired phosphors, such as red-emitting, greenemitting, andblue-emitting, to produce the finished screen.

This method of pattern application works well under some conditions butrequires strict controls and faithful adherence to the aforementionedschedule. Also, the rotational spreading of the slurry material becomesmore critical and more diflicult after the first pattern of elementalphosphor areas has been applied. Under these conditions the previouslyapplied elemental areas act as barriers to the flow of the secondarilyapplied slurry layer which is being caused to spread by the constantlychanging centripetal force imparted thereto by the rotation of thepanel. These barriers cause a condition known as spoking; i.e., bareareas which occur behind the previously applied dots because of themanner in which the secondarily applied slurry flows therearound. Also,it is extremely difficult to achieve a uniform area density of slurrymaterial under these conditions because of the vast difference in thevelocity of the slurry at the center of the panel at the start of thespinning schedule and at the edge.

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OBJECTS AND SUMMARY OF THE INVENTION It is an object of the invention toenhance the screening techniques utilized for the manufacture ofpatterned screen cathode ray tubes.'

It is another object of the invention to provide a method of achieving asubstantially uniform area density of slurry material prior to exposure.

It is yet another object of the invention to provide a method of screendeposition that is simple and economical to operate.

These objects are achieved in one aspect of the invention by theprovision of a method of forming a pattern on a prepared given area ofthe interior non-planar surface of a cathode ray tube face panel whichcomprises the steps of wetting at least the given area with a phosphorcarrying, radiation sensitized slurry without regard to the area densitythereof and then positioning the panel in a manner to achieve levigationand uniformity of area density of the slurry through the action of aconstant force. After the levigation the slurry coated panel is exposedthrough a patterned negative to a suitable source of radiation and isthen subsequently developed to remove the unexposed areas. For a threecolor cathode ray tube screen the above procedure is repeated twice morewith the radiation source offset to a new position relative to thescreen.

As used in this application, levigation shall mean the smoothing orevening out of a surface of a fluid or semifiuid material.

Utilization of this method provides enhanced cathode ray tube screensthat do not exhibit the degree of spoking found in screens made by priorart practices. It is simple and economical to operate and eliminates thecomplex tilting and spinning movements of prior methods. Also, itprovides an extremely uniform and easily reproducible area density forthe slurry, a condition of vast importance as a prelude to formingelemental pattern areas of uniform consistency and thus uniform lightoutput.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1-8 are diagrammaticrepresentations of various stages in the method;

FIG. 9 is a diagrammatic representation of another embodiment; and

FIG. 10 is a diagrammatic view of an exposure device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In referring now to thedrawings with greater particularity, in FIG. 1 there is shown a cathoderay tube face panel 10 of a type that might be used for a colortelevision tube. The panel 10 has a non-planar interior surface 12,which in this instance is concave, a skirt 14, and a given area 16 whichis to have the pattern formed thereon.

The panel 10 has given area 16 prepared for deposition by having itthoroughly cleaned and dried before commencement of the operation. Thegiven area 16 may also be pre-coated; e.g., with a layer of unsensitizedpolyvinyl alcohol.

Whether the panel is pre-coated or not the given area must be dry beforeproceeding to the next step. With the panel thus prepared it is placedin a substantially horizontal position with the concave surfaceuppermost and a quantity of a photosensitized slurry material 18 ispoured or otherwise dispensed thereinto substantially at the centerthereof.

A suitable slurry may comprise an aqueous suspension containing about 6weight percent of an organic, polymerizable binder such as polyvinylalcohol and about 8 weight percent (of the organic binder) of aphotosensitizer such as ammonium dichromate. To this suspension is addeda suitable quantity of phosphor having a relatively fine particle size;i.e., 2 to 3 microns. The quantity of phosphor will vary with thematerial, but the amount for phosphors currently being used will bebetween about :1 to :1 by weight of organic binder. After sufiicientmixing to assure a substantially homogeneous dispersion the viscosity ofthe slurry is adjusted by the addition of ethyl alcohol to within therange of 14 to 25 centistockes.

After the slurry has been dispensed into the face panel the panel istilted as is shown in FIG. 2 to allow the slurry to flow from the centerto at least the edge of the given area to accomplish the initial wettingaction. After the slurry has flowed to one edge of the given area thepanel is tilted in another direction to allow the slurry to flow toanother edge of the given area, as is shown in FIG. 3. This action isrepeated until all of the given area has been initially wetted.Depending upon the manner in which the slurry is dispensed into thepanel, the initial wetting may be accomplished by tilting the panel inthree or four directions; i.e., in relation to the points of a compass,the panel may be first tilted toward the north, thence toward the eastsouth east, and thence toward the west south west. If four directionsare necessary they may be north, east, south, and west; although notnecessarily in that order. The angle of tilt will be determined by theviscosity of the slurry and its flow characteristics and, of course, thetime limitations demanded by production quotas. For viscosities withinthe range of 14 to 25 centistokes an angle of tilt of 60 issatisfactory.

Alternatively, the panel may be rotatively mounted at a substantially 60angle and have the slurry dispensed thereinto. The panel is rotated at arelatively slow speed, say 3 to 4 r.p.m., until the given area isinitially wetted.

Whichever of the above methods of initially wetting the given area isutilized, after the wetting has been achieved the slurry is allowed tosubstantially repuddle near the center of the panel as shown in FIGS. 4and 5. The repuddling need not be complete and generally the perimeterof the puddle will be quite uneven, as may be seen in FIG. 5 where thecrosshatched section represents the wetted area and the stipped sectionrepresents the repuddled slurry.

After the repuddling has occurred the panel is inverted to the positionshown in FIG. 6 and allowed to remain substantially immobile until thelevigation of the slurry occurs. The levigation occurs in stages as theslurry begins to flow outwardly from the center of the panel toward theedges under the action of the constant force of gravity. The slurryundergoes flowing movement because of the concave interior surfaces ofthe panel.

Immediately subsequent to inversion the surface of the slurry may have arough and uneven texture such as is shown in FIG. 7; however, after apredetermined period of time the slurry levigates and assumes thesurface shown in FIG. 8. The area density of the slurry layer at thisstage will have substantial uniformity between any two points thereon;i.e., there will be on the average less than .5% variation between anytwo points. This kind of consistency was not possible by prior artmethods.

To provide this type of consistent performance, control of several otherparameters are necessary in addition to the viscosity of the slurry andthe time for levigation; namely, the temperature and the humidity of theatmosphere surrounding the panel during the slurry dispensing, wetting,and levigating.

For a slurry having a viscosity in the range of about 14 to 25centistokes and a phosphor-PVA ratio within the range set forth above,and a levigating time in the range of about 7 to 13 minutes, atemperature in the range of about 70 to 80 F. and a relative humidity inthe range of about 30% to 50% provide excellent results.

During the levigation stage it is desirable that all movement of air ordrafts be avoided since this might speed up the evaporation of thesolvents within the slurry mixture and cause it to set up with an unevensurface. This condition may be avoided by placing the panel within asubstantially confining enclosure during levigation. Some additionalcontrol may be had over the levigating of the slurry within theenclosure by providing a suitable hole or holes therein to controlconvection currents.

The amount of slurry dispensed into the panel is not particularlycritical within very broad limits. Obviously there must be an amount ofslurry sufircient to provide the desired area density over the givenarea, but any excess of this amount will not cause any unacceptableresults. This is so because the area density is dependent upon theviscosity of the slurry, the temperature, the humidity, and thelevigation time-all easily controlled parametersand, of course, thelevigation forcegravity which, within the relatively small verticaldifference between the panel center and edge, may be assumed to beconstant. Thus, any excess of the slurry will flow beyond the perimeterof the given area and may be wiped off after levigation has occurred. 4

Still another embodiment of the invention is shown in FIG. 9 wherein theslurry is hosed upwardly into an already inverted panel. One or morenozzles 20 may be utilized depending upon the size of the panel, andthey may be pivotally mounted as at 22 so that all of the given area maybe hosed on one pass of the nozzle. This procedure eliminates the stepsof wetting, repuddling, and inverting and provides the same results.

It is to be noted that, as used herein, hosing is meant to mean thefluid ejection of material by application of a fluid force or pressureand is to be differentiated from spraying which provides an aerated oratomized particle form of deposition.

Whichever of'the above methods of slurry application is utilized, afterthe levigation is complete the panel is mated with a suitable negative24 to provide a desired pattern and is positioned with an exposuredevice 26, as is shown in FIG. 10. A radiation source 28 is remotelyspaced from the panel and upon energization provides the radiationnecessary to polymerize the desired areas. After the exposure the panelis developed by washing it with a solvent for the unexposed slurry. Forslurries of the type described herein the washing solution may be water.

When the developing is finished the panel is dried and the cycle isrepeated. In the case of a three color tube the process is utilizedthree times.

This invention provides a substantial advance in the art of screeningcathode ray tubes. It eliminates the need for highly skilled operatorsand provides excellent uniformity of area density, a prime prerequisiteto manufacturing acceptable color picture tubes. Also, it lends itselfreadily to automatic procedures and, by elimination of the high speedspinning cycles, virtually eliminates the condition called spoking onthe second and subsequent slurry depositions. (The more slowly movingslurry has a better opportunity to flow around previously formedpatterns).

While there have been shown and described what are at present consideredto be the preferred embodiments of the invention, it will be obvious tothose skilled in the art that various changes and modifications may bemade therein without departing from the scope of the invention asdefined by the appended claim.

We claim:

1. In the method of forming a pattern on the concave lnterior surface ofa cathode ray tube face panel the steps comprising: wetting at leastsaid concave surface with a phosphor particle containing, organic,radiation sensitized, viscous slurry without regard to the area densitythereof by depositing a quantity of said slurry material substantiallyin the center of said concave surface when said surface is upwardlyoriented and in a substantially horizontal plane; angularly tilting saidsurface in at least three directions, one at a time, to substantiallywet at least said concave surface; returning said surface to saidhorizontal plane for a sufficient time to allow substantial repuddlingof said slurry material near the center of said surface, positioningsaid face panel with said concave surface downwardly oriented to achievelevigation and uniformity of area density of said material over saidconcave surface through the action of gravity; positioning a suitablenegative with respect to said phosphor coated surface; exposing saidslurry coated concave surface through said negative to a suitable sourceof radiation whereby portions of said slurry coated surface becomeselectively unremovable by a given solvent; and forming said pattern byutilizing said given solvent to remove that part of said slurry coatedsurface which has not become unremovable.

References Cited UNITED STATES PATENTS ALFRED L. LEAVITT, PrimaryExaminer J. R. BA'ITEN, JR., Assistant Examiner US. Cl. X.R.

11733.5 C, 33.5 CM, 33.5 CF

